Pets that deal physical damage will apply stacks, even if they don't apply on-hit effects or spell effects.

The very first instance of physical damage does not get the benefit of the armor reduction. The physical damage dealt is calculated first, and the debuff is applied after. This means the full benefit of the reduction is seen on the seventh hit, after all six stacks have been applied.

Armor reduction and penetration effects stack, allowing an attacking character to benefit simultaneously from Black Cleaver shred, penetration runes, other items such as Last Whisper, and champion abilities that circumvent armor.

When combined, The Black Cleaver and The Obsidian Cleaver will apply 2 stacks per instance of physical damage, instead of 1. This is because the unique passives are not named and they are not the same item. However, the two items share the same maximum stacks.

Multiple instances of physical damage applied at the same time (e.g. Zed'sRazor Shuriken with his shadow) will only apply one stack.

Strategy

Building multiple Black Cleavers can be viable: with Transcendence and 40% CDR, The Black Cleaver grants 64 AD and 400 health, and is now worth:

Trundle'sSubjugate reduces the armor of his target by 39.2% to 54.4% in the next 4 seconds (the effect does not apply any stacks). The armor reduction lasts for another 4 seconds after the drain completes.

Vi'sDenting Blows reduces the target's armor by 39.2% for 4 seconds after every third consecutive hit.